Simultaneous determination of 18α- and 18β-glycyrrhetic acid in human plasma by LC-ESI-MS and its application to pharmacokinetics

A highly sensitive and selective LC-ESI-MS was developed, validated for the simultaneous determination of 18α-glycyrrhetic acid (α-GA)and 18β-glycyrrhetic acid (β-GA) for pharmacokinetic studies in healthy subjects. Sample preparation was performed by liquid-liquid extraction with ethyl acetate and the separations were achieved using a C₁₈ column with the mobile phase composed of 10 mmol/L ammonium acetate solution-methanol-acetonitrile (40:36:24, v/v/v)at a flow rate of 1 mL/ min. The internal standard was honokiol and the epimers were quantified using a single quadrupole mass spectrometer employing ESI in the negative ion mode. The separation factor, α, was 1.512 for α-and β-GA. The standard curves were linear for both epimers with coefficients of determination (r≥0.9998) over the concentration range of 1-150 ng/mL. The precision and accuracy were ≤4.33 and ≤6.57%, respectively. The mean plasma extraction recoveries were 82.23 ± 1.91 and 84.29 ±2.09% for α-GA and β-GA, respectively. The assay was successfully applied to evaluation of the pharmacokinetic properties of α-GA and β-GA from 19 volunteers who had received oral administration of diammonium glycyrrhizinate capsules. The initial data suggest that glycyrrhizin metabolizes to glycyrrhetic acid fairly slowly and the elimination of α-GA is slower than that of β-GA.